The present invention relates to a method and an apparatus for manufacturing a positive electrode foil of an aluminum electrolytic capacitor.
Aluminum electrolytic capacitors are widely used in the circuits of electronic apparatuses. A common type of aluminum electrolytic capacitor comprises a capacitor element having a positive electrode foil and a negative electrode foil separated by a separator and wound to a roll, a driving liquid electrolyte permeated into the capacitor element, and a metal case accommodating the capacitor element.
The positive electrode foil is conventionally manufactured by the following procedure. An aluminum foil is etched to increase its effective surface area and coated by anodizing process with a dielectric oxide film. The effective surface area is controlled with determining the etching conditions, Controlling an applied voltage in an anodizing process determines a thickness of the film, and thereby, a rated voltage and a capacitance of a finished capacitor can be controlled.
FIG. 7 is a flowchart of a conventional procedure of manufacturing the positive electrode foil. In the etching process, an aluminum foil 71 having a thickness of 0.05 to 0.11 mm and a length of about 5000 m provided in a coil form is continuously etched by the etching operation 72 in an etching apparatus. The apparatus is equipped with a control panel 73 manually preset with the etching conditions for a desired effective surface area. The etched foil 74 is rolled in every 1,000 m into a coil specified with a lot number. Each lot of the etched foil 74 is then subjected to a characteristic inspection 75 (conforming to EIAJ RC-2364A).
In general, a number of the etching apparatuses are used for manufacturing the etched foils 74 having the same effective surface area.
In the anodizing process, each etched foil 76 is continuously impressed with a constant voltage in the anodizing operation 77 in an anodizing apparatus to obtain a dielectric oxide film, i.e., an anodized foil 79. The anodizing apparatus is equipped with a control panel 78 preset manually with the anodizing conditions based on the rated voltage and the desired effective surface area of the etched foil 74. The anodizing conditions may include predetermined levels (data from the preceding experiments and production) of applied voltage and current and temperatures in the anodizing process.
The anodized foil 79 is then subjected to a characteristic inspection 80 (conforming to EIAJ RC-2364A) and a judgment 81 before a finished positive electrode foil 82 is released.
If, in the characteristic inspection 80, it is judged that the resultant rated voltage and capacitance are not desired values, the anodizing conditions are updated to obtain them.
FIG. 8 illustrates such an anodizing apparatus. The anodizing apparatus is fed with an etched foil 91 generally having a thickness of 0.05 to 0.11 mm, a width of 500 mm, and a length of 500 to 1,000 m. The etched foil 91 is passed over a feed roller 92 connected to the positive electrode of a first direct-current source 97 and introduced by a running roller 93 into an anodizing tank 96. The anodizing tank 96 has a plurality of cathode electrodes 95 arranged therein in parallel with the etched foil 91 to be passed and is filled with an anodizing processing liquid. In the anodizing tank 96, the etched foil 91 serving as the positive electrode is anodized to obtain a dielectric oxide film thereon. The etched foil 91 coated with the dielectric oxide film is pulled out from the anodizing tank 96, and then, conveyed via a depolarization tank 98 to a first re-anodizing tank 99 and a second re-anodizing tank 101. Each of the re-anodizing tanks 99 and 101 has a plurality of cathode electrodes arranged therein and connected to a second direct-current source 100 for a continuous re-anodizing process of the etched foil 91. Finally, the etched foil 91 is passed through a post-process tank 102 and rolled for obtaining a roll of a positive electrode foil 103. The voltage of the second direct-current source 100 is determined based on a measured voltage detected with a voltage-detecting roller 104 provided before the roll of positive electrode foil 103.
Another anodizing process is disclosed in Japanese Patent Laid-open Publication No. 2-128415. When impressed with different levels of the anodizing voltage in steps, an aluminium foil is depolarized in a certain period between any two steps of the anodizing. This will minimize a leakage current on the positive electrode foil.
Also, a manufacturing apparatus disclosed in Japanese Patent Laid-open Publication No. 58-197713 includes a pair of current electrode and voltage electrode sandwiching the electrode foil in a processing liquid. While the current electrode is fed with an alternating current, the voltage at the voltage electrode is measured by a phase sensitive voltmeter and used for controlling the anodizing process.
The conventional method of manufacturing the positive electrode foil 82 however causes the effective surface area of the etched foil 76 to vary between the etching apparatuses. This hardly ensures a constant capacitance in any capacitor with the positive electrode foil 82 manufactured from an aluminum foil 76 by the anodizing process 77 even under the predetermined anodizing conditions. That is because the dielectric oxide film is not uniform in the thickness due to a change of the current for the anodizing process 77. The change is caused by variations of the effective surface area of the foil 76.
Therefore, the step of judgment 81 is provided after the step 80 for inspecting the properties of the anodized foil 79 in the beginning of the anodizing process. When the judgment 81 is negative, the anodizing conditions are modified and re-set. This hence produces a loss in the time for manufacturing the anodized foil 79.
For continuously manufacturing the positive electrode foils 82 of different rated voltages with a single anodizing apparatus, an aluminum foil is inserted between the anodized foils 79 of different voltages for inhibiting the anodizing conditions for the preceding lot from affecting to the succeeding lot. This makes the procedure more difficult, and in worse case, produce defectives of the anodized foil 79.
Moreover, as the anodizing conditions are set by an operator, they may often have artificial errors.
The etched foil 76 is varied in the effective surface area along the lengthwise direction. This will hardly be overcome by the anodizing conditions calculated in each lot. In the conventional anodizing apparatus shown in FIG. 8, while the output of the second direct-current source 100 is controlled based on a measured voltage detected by the voltage sensor roller 104, the voltage in the tank 99 may vary corresponding to the specific resistance of the etched foil 91 between the first re-anodizing tank 99 and the voltage sensor roller 104. As a result, the positive electrode foil 103 is hardly uniform in the capacitance and the withstanding voltage along the lengthwise direction.
In the method disclosed in Japanese Patent Laid-open Publication No. 02-128415, the aluminum foils is impressed with different levels of the anodizing voltage at multiple steps, and more specifically, with a constant anodizing voltage at each tank for the anodizing process. Consequently, the withstanding voltage of the finished anodizing film can hardly be examined. As a result, a capacitor employing the positive electrode foil may hardly be uniform in the capacitance and the withstanding voltage.
In the apparatus disclosed in Japanese Patent Laid-open Publication No. 58-197713, the electrode foil is measured at a particular portion for measuring anodizing conditions using a pair of the current electrode and the voltage electrode. And thus, the anodizing condition is hardly controlled throughout the length based on the measurement at the particular position.
A method of manufacturing the positive electrode foil of an aluminum electrolytic capacitor is provided where anodizing conditions for an etched foil are optimally determined and automatically registered to minimize variations in the capacitance and the withstanding voltage along both lengthwise and widthwise directions of the positive electrode foil. And hence, it ensures the product quality and increases the production efficiency.
In the method, the current applied for developing a dielectric oxide film on the etched foil in an anodizing process is determined to be optimum for the effective surface area of the etched foil.
The method consists mainly of an etching process and an anodizing process. The etched foil produced in the etching process is subjected to a constant current inspection, and then, the anodizing conditions are calculated from the result of the constant current inspection and transferred to a control panel for an anodizing tank for setting the anodizing apparatus.
As a result, the anodizing conditions optimum for the etched foil can automatically be registered and used for the anodizing process of the foil.
Another method of manufacturing the positive electrode foil is provided where an etched foil is fed with a current from a direct-current source and conveyed between cathode electrodes provided in an anodizing tank by running rollers for the anodizing process. While the running rollers are made of metallic material, a voltage sensor is connected between one of the running rollers and the cathode electrodes. A voltage measured by the voltage sensor is fed back to the direct-current source for controlling its output voltage.
This method allows the output voltage of the direct-current source to be controlled through measuring the withstanding voltage of a dielectric oxide film developed on the etched foil in the anodizing tank. Accordingly, as the withstanding voltage of the dielectric oxide film remains constant, unwanted variations in the properties of the positive electrode foil including the capacitance and the withstanding voltage along both lengthwise and widthwise directions can be minimized.